1. Field of the Invention
This invention relates to a current controlled variable attenuator circuit using a current controlled resistor and more particularly to a current controlled resistor means for use in a high-fidelity hearing aid. The current controlled variable attenuator circuit limits input signal levels received from a hearing aid microphone to prevent overload of the hearing aid output amplifier or to prevent discomfort to the hearing aid user. The current controlled resistor means also provides a current controlled variable resistor useful in other applications requiring a broader linear response than presently available from saturated bipolar transistors used as current- or voltage-controlled resistors.
2. Background of the Prior Art
One prior art approach can be seen in Killion, U.S. Pat. No. 4,170,720. That particular hearing aid used a field effect transistor (FET) in shunt with the microphone output terminal of a hearing aid. The FET was used to limit the sound level input to prevent the linear output capabilities of the hearing aid amplifier from being exceeded, or to prevent the sound level pressure delivered to the user's ear from being uncomfortable. While the FET could handle input signal levels of from 50 to 100 millivolts with adequate linearity, suitable FET devices are not readily available in conventional bipolar circuits such as are typically used in hearing aids. A saturated bipolar transistor can be used as a variable attenuator, but above a 3 millivolt input signal level the output of a bipolar transistor attenuator become non-linear as described in detail in the aforementioned patent.